1. Field of the Invention
This invention relates to a method for producing size selected particles, and more particularly this invention relates to a co-precipitation method for consistently producing particles within a predetermined size from a fluid containing relatively tiny and huge particles.
2. Background of the Invention
Certain sized particles as electrode active materials for secondary batteries, or as catalysts for chemical reactions, can optimize the performance associated with their applications. However, consistent generation of uniform sized particles, and the uniformed sized particles themselves remain elusive. This is because particles under one micron easily coagulate, aggregate, or associate with each other irregularly. Particle aggregation refers to formation of clusters in a colloidal suspension and represents the most frequent mechanism leading to unwanted particle growth. During this process, which normally occurs within short periods of time (seconds to hours), particles dispersed in the liquid phase stick to each other, and spontaneously form irregular particle clusters, flocs, or aggregates. As aggregation proceeds from early to later states, the aggregates grow to size of 1-100 micron, depending on the reagents used and the reaction method.
Efforts have been made to produce and maintain particles below 20 microns. Batch and continuous reactors have been part of these efforts.
Particle sizes of electrode active material precursor and electrode active material produced during co-precipitation using conventional continuous stirred tank reactor (CSTR) vary widely from a few nanometers to several dozen micrometers. This varying particle size lowers tap density and reduces the performance of lithium secondary batteries. For example, tiny particles (e.g., less than 500 nm in diameter) increase the total surface area of electrode active materials. This in turn leads to a decrease in the cycle life of the battery due to side reactions with electrolyte on the high surface area of the small particles.
Conversely, very large particles (e.g., more than 40 μm) cause problems with cathode coatings and create short circuits in the batteries.
Sieving processes have been used to produce specific sized particles. However, sieving does not eliminate particles at the small end of the spectrum. In addition, separated large particles are disposed of as an off-spec secondary waste stream.
Sedimentation methods have been employed to eliminate tiny particles, but these methods require time and several repetitions. Air-classification has also been used to separate dried particles in certain size ranges. These processes involve cycloning whereby dried powder materials are subjected to centrifugal force and therefore particle collision and rotor blade collision. This leads to particle loss and particle damage.
Batch reactors have been used to produce similar particle sizes. However, uniformity of particle sizes between batches is hit or miss. Specifically, average particle size, particle size distribution and quality of particles generated via batch processes fluctuate more than is acceptable.
A need exists in the art for a method to produce specific sized electrode active material precursor and electrode active material precursor without tiny and huge particles. The method should consistently produce uniform sized particles so as to optimize the tap density of the particles being produced. The method should incorporate common materials processing protocols.